The present invention relates to stents, and more particularly, to stent grafts having an expandable web structure configured to provide enhanced embolic protection and reduce restenosis and thrombus formation.
Stents are commonly indicated for a variety of intravascular and non-vascular applications, including restoration and/or maintenance of patency within a patient""s vessel. Stents are also used to reduce restenosis of a blood vessel post-dilation, thereby ensuring adequate blood flow through the vessel. Previously known stents are formed of a cell or mesh structure, having apertures through which endothelial cells migrate rapidly. These endothelial cells form a smooth coating over the stent that limits interaction between the stent and blood flowing through the vessel, thereby minimizing restenosis and thrombus formation.
In many applications, in addition to maintenance of vessel patency and limitation of restenosis, protection against release of embolic material from the walls of the vessel is desired. Emboli released into the bloodstream flow downstream, where they may occlude flow and cause death, stroke, or other permanent injury to the patient. The apertures between adjoining cells of previously known stents may provide an avenue for such embolic release, depending upon the application.
In addition to embolic protection, a smooth surface, i.e. a substantially continuous surface lacking apertures, may be desired to permit unencumbered recrossability with guide wires, balloon catheters, etc., into the lumen of the stent, for example, to compress stenosis or restenosis and open the lumen, to resize the stent to accommodate vascular geometry changes, etc. Further, equalization of forces applied by or to the stent may be desired to reduce a risk of the stent causing vessel dissection. Due to the apertures, previously known stents may provide only limited embolic protection, recrossability, and force distribution in some applications.
A covered stent, or a stent graft, comprises a stent that is at least partially externally-covered, internally-lined, or sintered with a biocompatible material that is impermeable to stenotic emboli. Common covering materials include biocompatible polymers, such as Polyethylene Terephthalate (PETP or xe2x80x9cDacronxe2x80x9d) or expanded Polytetrafluoroethylene (ePTFE or xe2x80x9cTeflonxe2x80x9d). Stent grafts may be either balloon-expandable or self-expanding. Balloon-expandable systems may be expanded to an optimal diameter in-vivo that corresponds to the internal profile of the vessel. Upon compression, self-expanding embodiments characteristically return in a resilient fashion to their unstressed deployed configurations and are thus preferred for use in tortuous anatomy and in vessels that undergo temporary deformation.
A stent graft provides embolic protection by sealing emboli against a vessel wall and excluding the emboli from blood flow through the vessel. Additionally, since the biocompatible material of a stent graft closely tracks the profile of the stent, forces applied by and to an impinging vessel wall are distributed over a larger surface area of the stent, i.e. the force is not just applied at discrete points by xe2x80x9cstrutsxe2x80x9d located between apertures of the stent. Rather, the biocompatible material also carries the load and distributes it over the surface of the stent. Furthermore, stent grafts provide a smooth surface that allows improved or unencumbered recrossability into the lumen of the graft, especially when the biocompatible material lines the interior of, or is sintered into, the stent.
While the biocompatible materials used in stent grafts are impermeable to, and provide protection against, embolic release, they typically do not allow rapid endothelialization, as they also are impermeable or substantially impermeable to ingrowth of endothelial cells (i.e. have pores smaller than about 30 xcexcm) that form the protective intima layer of blood vessels. These cells must migrate from the open ends of a stent graft into the interior of the stent. Migration occurs through blood flow and through the scaffold provided by the graft. Such migration is slow and may take a period of months, as opposed to the period of days to weeks required by bare (i.e. non-covered) stents.
In the interim, thrombus may form within the lumen of the graft, with potentially dire consequences. As a further drawback, migration of the endothelium through the open ends of a graft may leave the endothelial coating incomplete, i.e. it does not span a mid-portion of the graft. In addition, the endothelial layer is often thicker and more irregular than the endothelialization observed with bare stents, enhancing the risk of restenosis and thrombus formation.
Porous covered stents also are known. For example, U.S. Pat. No. 5,769,884 to Solovay describes a covered stent having porous regions near the end of the stent, wherein the pores are sized to allow tissue ingrowth and endothelialization. The middle region of the stent is described as being much less porous or non-porous, to encapsulate damaged or diseased tissue and inhibit tissue ingrowth.
The Solovay device is believed to have several drawbacks. First, the end regions of the stent are described as having a preferred pore diameter as large as 120 xcexcm. However, pore diameters greater than about 100 xcexcm may provide inadequate embolic protection; thus, if the end regions compress a stenosis, hazardous embolization may result. Second, since the middle region of the stent is adapted to inhibit tissue ingrowth, endothelial cells must migrate into the middle region of the stent from the end regions and from blood flow. As discussed previously, such migration is slow and provides an inferior endothelial layer.
An additional drawback to previously known devices is that many are not configured for use at a vessel bifurcation. A bare stent placed across a vessel side branch is expected to disrupt flow into the side branch and create turbulence that may lead to thrombus formation. Conversely, placement of a non-porous covered stent/stent graft across the bifurcation is expected to permanently exclude the side branch from blood flow, as such grafts are substantially impermeable to blood.
In view of the drawbacks associated with previously known stents and stent grafts, it would be desirable to provide apparatus and methods for stenting that overcome the drawbacks of previously known devices.
It further would be desirable to provide methods and apparatus that reduce the risk of embolic release, while also reducing the risk of restenosis and thrombus formation.
It also would be desirable to provide apparatus and methods for stenting that allow improved recrossability into the lumen of the apparatus.
It would be desirable to provide apparatus and methods for stenting that distribute forces applied by or to the apparatus.
It still further would be desirable to provide apparatus and methods suitable for use in bifurcated vessels.
In view of the foregoing, it is an object of the present invention to provide apparatus and methods for stenting that overcome the drawbacks of previously known apparatus and methods.
It is an object to reduce the risk of embolic release during and after stenting, and also reduce the risk of restenosis and thrombus formation.
It is yet another object of the present invention to provide apparatus and methods that allow unencumbered recrossability into the lumen of the apparatus.
It is an object to provide apparatus and methods for stenting that distribute forces applied by or to the apparatus.
It is an object to provide apparatus and methods suitable for use in a bifurcated vessel.
These and other objects of the present invention are accomplished by providing apparatus comprising a stent, for example, a balloon-expandable, a self-expanding, a bistable cell, or a metal mesh stent. A biocompatible material at least partially is sintered between the apertures of the stent, or covers the interior or exterior surface (or both) of the stent. Unlike previously known stent grafts, embodiments of the present invention are both permeable to endothelial cell ingrowth and impermeable to release of critical-sized emboli along their entire lengths. Thus, the present invention provides the embolic protection, force distribution, and improved recrossability characteristic of non-porous stent grafts, while further providing the protection against restenosis and thrombus formation characteristic of bare stents.
In one preferred embodiment, the biocompatible material of the present invention comprises, for example, a porous woven, knitted, or braided material having pore sizes determined as a function of the tightness of the weave, knit, or braid. Pore size is selected to allow endothelial cell ingrowth, while preventing release of emboli larger than a predetermined size. In an alternative embodiment, the biocompatible material comprises pores that are chemically, physically, mechanically, laser-cut, or otherwise created through the material with a specified diameter, spacing, etc. The pores may be provided with uniform or non-uniform density, size, and/or shape. The pores preferably have a minimum width large enough to promote endothelial cell ingrowth, and a maximum width small enough to reduce the risk of embolic release.
Apparatus also is provided for use in a bifurcated or branched vessel. Since the porous biocompatible material of the present invention is permeable to blood flow, it is expected that, when implanted, flow into a side branch will continue uninterrupted. The small diameter of the pores, relative to the diameter of the stent apertures, will provide a grating that is expected to minimize turbulence and allow thrombus-free blood flow into the side branch. Optionally, the porosity, i.e. the diameter, density, shape, and/or arrangement, of the pores may be altered in the region of the side branch to ensure adequate flow.
Alternatively, the stent and biocompatible material may comprise a radial opening. When stenting at a vessel bifurcation or branching, the radial opening may be positioned in line with the side branch to maintain patency of the branch. Alternatively, a plurality of radial openings may be provided along the length of the implant to facilitate continuous blood flow through a plurality of side branches.
Stents for use with apparatus of the present invention preferably comprise a tubular body with a wall having a web structure configured to expand from a contracted delivery configuration to an expanded deployed configuration. The web structure comprises a plurality of neighboring web patterns having adjoining webs. Each web has three sections: a central section arranged substantially parallel to the longitudinal axis in the contracted delivery configuration, and two lateral sections coupled to the ends of the central section. The angles between the lateral sections and the central section increase during expansion, thereby reducing or substantially eliminating length decrease of the stent due to expansion, while increasing a radial stiffness of the stent.
Preferably, each of the three sections of each web is substantially straight, the lateral sections preferably define obtuse angles with the central section, and the three sections are arranged relative to one another to form a concave or convex structure. When contracted to its delivery configuration, the webs resemble stacked or nested bowls or plates. This configuration provides a compact delivery profile, as the webs are packed against one another to form web patterns resembling rows of the stacked plates.
Neighboring web patterns are preferably connected to one another by connection elements preferably formed as straight sections. In a preferred embodiment, the connection elements extend between adjacent web patterns from the points of interconnection between neighboring webs within a given web pattern.
The orientation of connection elements between a pair of neighboring web patterns preferably is the same for all connection elements disposed between the pair. However, the orientation of connection elements alternates between neighboring pairs of neighboring web patterns. Thus, a stent illustratively flattened and viewed as a plane provides an alternating orientation of connection elements between the neighboring pairs: first upwards, then downwards, then upwards, etc.
As will be apparent to one of skill in the art, positioning, distribution density, and thickness of connection elements and adjoining webs may be varied to provide stents exhibiting characteristics tailored to specific applications. Applications may include, for example, use in the coronary or peripheral (e.g. renal) arteries. Positioning, density, and thickness may even vary along the length of an individual stent in order to vary flexibility and radial stiffness characteristics along the length of the stent.
Stents for use with apparatus of the present invention preferably are flexible in the delivery configuration. Such flexibility beneficially increases a clinician""s ability to guide the stent to a target site within a patient""s vessel. Furthermore, stents of the present invention preferably exhibit high radial stiffness in the deployed configuration. Implanted stents therefore are capable of withstanding compressive forces applied by a vessel wall and maintaining vessel patency. The web structure described hereinabove provides the desired combination of flexibility in the delivery configuration and radial stiffness in the deployed configuration. The combination further may be achieved, for example, by providing a stent having increased wall thickness in a first portion of the stent and decreased wall thickness with fewer connection elements in an adjacent portion or portions of the stent.
Embodiments of the present invention may comprise a coating or attached active groups configured for localized delivery of radiation, gene therapy, medicaments, thrombin inhibitors, or other therapeutic agents. Furthermore, embodiments may comprise one or more radiopaque features to facilitate proper positioning within a vessel.
Methods of using the apparatus of the present invention also are provided.